Most film cartridges (also called film magazines) incorporate some kind of fabric or woven material in their assembly as a means of preventing light from entering the film exit in the cartridge. This guards against premature exposure of the outer convolutions of the film. A common fabric incorporated as a light lock material is referred to as "velvet". Velvet is a nylon yarn fiber which is bundled and woven into a rayon fabric backing material.
Assembly of velvet light lock material to conventional metal film cartridges is accomplished by applying heat to both lips of the cartridges which have been formed by a die set (but are still in a multiple strip stage yet to be died out). The velvet is pre-slit to size and applied in continuous web form to both sides of the multi cartridge strip in the proper locations. The velvet web is then clamped under pressure while the heat from the metal cartridge strip activates an adhesive coated on the back of the rayon fabric of the velvet, attaching the rayon backing of the light lock web to the cartridge strip. Once the adhesive has cured, the cartridge strip is indexed to the next step in the process. There the two velvet webs are cut between the individual notches denoting each cartridge by a hot knife process. This process is satisfactory as long as the outer shell of the cartridge is made of sheet steel.
However, a new polystyrene cartridge is being developed as a consumer product in the 35 millimeter film range. One of the main requirements of this new product is that the cartridge shell be made of plastic instead of sheet steel. This requires a whole multitude of design changes to achieve automated assembly of light lock material to film cartridges.